The morphologic determinants of mitral systolic anterior motion (SAM and subaortic obstruction in hypertrophic cardiomyopathy (HC) are not completely understood. In the present study mitral valve specimens from 43 patients with HC and basal outflow obstruction (produced by systolic contact between mitral leaflet and ventricular septum) were utilized to relate morphometric measurements of leaflet area to certain morphologic and functional assessments of left ventricular outflow tract geometry and valvular motion obtained from echocardiograms in the same patients. Twenty-four patients (56%) had mitral valves of normal size (leaflet area <12.0 cm2) and 19 patients (44%) had enlarged and elongated valves (area > 12.0 cm2). Compared to normal sized mitral valves, the enlarged valves were situated more posteriorly in a larger left ventricular outflow tract (cross-sectional area 3.3 +/- 1.0 vs. 1.9 +/- 0.7 cm2 for normal sized valves; p< 0.001), and also had greater systolic excursion of anterior leaflet (16.2 +/- 4.5 mm vs 13.3 +/- 3.3 mm; p< 0.02) with a distinctive sharp angled bend and localized contact of leaflet tip with ventricular septum. Normal sized mitral valves had a different mechanism of outflow obstruction with more restricted SAM (with little of no bending).